Current pulse generator



Sept. 19, 1961 KAISER ETA], 3,001,091

CURRENT PULSE GENERATOR Filed March 12, 1958 FIG. I.

so 20 4a -4 5 1 l 40 a g 30 H/ E I 6 H l l I4 38 44 3 72 1 L .A AV 62 o g s [0 34 W l 42 60 I I 32 l6 I2 36 A 50 64 66 Z3 i/gz T -54 /68 FIG FIG. 3.

- T "I PERIOD INVENTORS ARTHUR KAISER I CARL. TISHLER JW W ATTO R N EY 3,001,091 CURRENT PULSE GENERATOR Arthur Kaiser, Trumbull, and Carl Tishler, Bridgeport, Conm, assigno'rs to Sperry'Rand Corporation, New York, N.Y., a corporation of Delaware Filed Mar; 12, 1958, Ser. No. 721,005 Claims. (Cl. 307-885) This invention relates to current pulse generators and more particularly to a transistor current pulse generator capable of producing relatively high current pulses having a flat pedestal, short rise and decay times, and small storage periods.

' In many situations it is necessary to provide circuits for driving a low impedance load with .a high current pulse, such circuits being required to switch a given value of current through a low impedance at a given speed and duty cycle. For example, a circuit for driving an inhibit plane of a coincident current ferrite core memory may be required to switchseveral-hundred milliamperes through a load consisting of a small resistance in series with a relatively large inductance. I

Heretofore, to. provide such a circuit wherexthe active network elements are transistors has required alarge number of such transistors in a circuit, generally in parallel arrangement, each of the transistors drawing large quiescent currents. Also, these circuits have not been indepedent of transistor parameter variations.

Accordingly, it is the primaryobject of this invention to provide a transistor circuit capableof producing relatively high amplitude current pulses which-have desirable characteristics, such as fastrise and fall times, relatively fiat pedestals, and negligible storage effects;

It is a further object to provide a transistor circuit as in the preceding object wherein there is a negligible current drain from power supplies during quiescent periods and which is substantially independent of transistor parameters. 1 v

Generally speaking, in accordance with the invention, there is provided a circuit comprising at least one transistor amplifier wherein there may be driven a current pulse having a chosen amplitude, duty cycle, pulse repetition frequency, and rise and decay times through an inductive load having a predetermined value of inductance. There is included in series with the inductive load, a parallel combination of a resistance-and a capacitance, the value of the capacitance being so chosen as to provide a critically damped condition. The transistor utilized in this circuit preferably has a power dissipation rating which is substantially equal to the transit time dissipation plus the saturation. The voltage rating of the employed transistor is; preferably equal to the product of the current through the load and the sum of the series resistances in :the output. a r v For a better understanding of the invention, together "with other and further objects thereof, reference is had to the following description taken together with'the accompanying drawings and its scope will be pointed out in the appended claims. I

In the drawings, FIG. 1 is a schematic depiction of an embodiment of the invention which employs a cas- .caded arrangement of three transistors;

FIGS. 2 and 3 are diagrams conveniently utilized in explaining the operation of the circuit of FIG. 1; and

FIG. 4 is a theoretical equivalent circuit of the output stage of the circuit in FIG. 1.

Referring now to FIG. 1, there is shown an NPN transistor havingan emitter 12,. a collector-14, and

a base- 16. Transistor 10 is connected in thec om'n'ion emitter configuration, positive potential bein'gapplied to collector 14 from a potential source 18 through a resist- States Patent 0 ance 20. Base 16 is returned to a source of negative potential 22 through a resistance 24. In series with base 16 is a parallel combination 28 comprising a capacitance 30 and a resistance 32. In the quiescent state with chosen biasing voltages from sources 18 and 22, transistor 10 is maintained at cutoff and upon the application of: a positive pulse of a proper value to base 16, transistor 10, is driven into conduction'at saturation. The values of resistance 32 andcapacitance 30 are so chosen that they convert the input pulse to one having a relatively steep exponential decay, the lowest amplitude of such decay still exceeding the amount required to cause transis: tor 10 to conduct at saturation. The negative potential applied to base 16 from source 22 is provided to aid in maintaining transistor 10 at cutoff during quiescent periods, and also to minimize the efliect of any 'D.C. restoration effects due to the action of capacitance 30; It is to be noted that with the application to base 16 of a positive pulse whose leading edge greatly exceeds in amplitude the amount required to drive transistor 10 into conduction at saturation, advantageously sharp and short rise times are obtained inthe output thereof. Also, the steep exponential decay caused by the action of parallel combination 28 serves to eliminate most of the deleterious effects due to the phenomenon of charge storage delay. f The output from collector14, which is a pulse with a relatively flat pedestal, is applied to the base 36 of a PNP: transistor34 through a parallel combination 38 of a capacitance 40 and a resistance 42, the values of the:

. components of parallel combination 38 being chosen to cause the same effect as that of parallel combination 28, i.e., the conversion of the flat negative output pulse'frorn. collector 14 to a pulse having a relative steep exponential, decay. The collector 44 of transistor 34 has negativej biasing potential from source 46 applied thereto through;

a resistance 48, its emitter 50 is connected to a refer'ehce potential source, i.e., groundvin the present illustration. and base 36 has positive potential from a source 54 applied, thereto through a resistance 52. Positive potential source.

54 has the same purpose as negative potential source 22 in.

connection with base 16 of transistor 10, i.e., the assist-v ing in maintaining transistor 34 at cutoff during quiescent states, and also to minimize D.C. restoration effects caused by the. action of capacitance 40. Similar to the.

- input pulse -to base 16 oftransistor 10, the amplitude dissipation across the transistor when it is conducting at of the input pulse to base 36 of transistor 34 is chosen to have value much greater than that required to; drive, transistor 34 into saturation from cutoff to insure short rise times of the output pulse from collector 44. The values of the components of parallel combination 38 are also chosen such that its peaking effect does not cause the input to base 36 to drop below the value necessary to maintain transistor 34' at conduction at saturation. It is seen that the series cascading arrangement of NPN transistor 10 and PNP transistor 34 serves advanta geously in that during quiescent periods, both transistors are non-conductive, thereby resulting in a negligible power drain and both transistors are rendered simultaneously conductive with the application of a positive input pulse to the circuit.

The positive output pulse from collector 44 is applied through a parallel combination 56 comprising a capacitance 58 and a resistance 60 to the base 64 of an NPN transistor 62. The emitter 66 of transistor 62 is in the grounded configuration and. base 64 is returned to a source of negative potential 68 through a resistance 70, Positive biasing voltage is applied to the collector 72 of transistor 62 from potential source -74 through the load 76 1 and a parallel combination 78 of a resistance 80 and a capacitance 82. As with parallel combinations 28 and 38, parallel combination 56 serves to peak the positive Patented Sept. 19, 1961 aal rectangular outputpu lse 'irom collector 44 to one having at saturannn,

off tlie preceding transistors in the circuit is provided to assist in maintaining transistor 62 at cutoff during its qui 'cent stat'e andro substantially eliminate an DIl. restorationefiects caused by thej'op'erationof capacitance 58. Transistor 62is 'of'th'e NPNftype, so that thethree tia istors inthe circuit can simultaneously 'be substanl isshown in'FlG. 1, the "102521 'depicted'i's "an inductance 8 4 inser'ieswith a resistance 86. Such depiction is'utililie 'to conveniently illustrate an inductive load with its rsistancecornponent. j

icon'si dering 'the operation of the circuit, it" isseen that the application of arectang'ular positive pulse through parallel combination '28 res'ults in a 'peakedfpuls'e being applied tojbase 1 6. The "output atcollector l l is a'neg'ati'ieepulse having "a relatively fiatipede'stal, since du'rin'g theentireperiod of application or the pulse to "base 16,

transistor, "as conducted, at "saturation. The negative cjit ut tram collecter 14 eaked by parallelcombination 3'8, "and a pliedre h'ase 36, drives transistor "34 into saturationrconduction throughout the duration of "the pulse, The 'fiatpositive cutput ulse from *colleeto'r 44, is slntila'rly' peaked by the parallel e'oinbination "56, and

, v transistor 6 2 ednductsat'saturation durinfgthe'time'ofthe ifil lieatien of thefpuls'e thereto'so thatthe current'jplils'e d r njthrough load 763s "a rectangular 'c'ui-rentpulse 'flatfpede'stal andjhavin'gan'an plitudejdetermined by the voltage {source '74, load 76, resistance 80"and the staturationi'resistance of transisto 62. This'eut ut'cnrrent 's'eii's'al'so one having"relatively"short' rise and decay 'du e'to' the factthat the input.'pu1se'to' each transistor leading edge greatly in e'X'cessof thearrfount'required j d'thof thi's output pulse substantiallycorrespondsto the"peaking circuits. The base biasing voltages are ease-en to be large enough to overcome the 'voltageswhich de lop "across 'c'apacitances 30, 40, '58 respectively due t 'DLC, I restoration affects 'athigh pulserepetitionj'rates.

e Re tances 24,52 and '70 are 'signalis'olation resistors yvhieh'far'e chosen to be large enough'not'todraw appreciable signal currents 'from'thebase "circuits but small enough "to establish proper D.C. base 'bi'asec'l currents, Resistance 80 andcapacitance' 82fp'rovidea cernpe'nsat'ing dircuit which im'ini'mizes theinductance effects of load and'the basis or selectionefpropervalues' therefor will be'furthe'r deyeloped hereinbelow.

'Iypiealcircuit values for the circuit 'of FIG. 1 are as o llows: capacitance '30, 0.002 'microfarad, resistance :32, "470 chins, "potential source '22, minus 2fvolts, potential sourc 18, 'IO VOItsQresistanCe 20, l50ohms, capacitance 40; 0.03 microfaradfresistance '42, 33 'bhnis, resistance 48, 82 'fohrns, potential "source, '4 6frn'inus "volts, potential source 54,2 volts,capacitance 58, 0.03 microfarad' resis tance 60, 33 ohms, potential source 68,1minus2 volts, resistance 80, 150 ohms, capacitance 82, 0.0024 microfared, potentialso'urce "68, minus' 2 volts, and resistances 24,52, and 70f3'30'0h1ns each. The load simulates an inductive load having an inductance value of 24 '1'nicr'ohenry'anda resistancevalue of 10 ohms. The transistor 62 may typically be one such as is designated 2N385. It is to beunderstodd that'the component values stated herein are applicableto a particular design application and it is-notrintendedthat the'jinvent'icn be limited thereby,

' nveithe're'spective transistor into{saturation and the r a width of'the in utpulsetdthe circuit due'tothe'vin t'uar'elimination of fch'ar g'e etora'ge'effects by the use of v t cutofi during thequi'escent periods in the circuit and are "simultaneou ly rendered conduetive at fsatura tie "withrheap lication of'a current, pulse of suitable Utilizing the circuit values given in the preceding para- "graph, a "rectangular currentpulse having an amplitude of 10 milliampere when applied through parallel combination 28 decays to 1 milliarnpere at the base 16 of transistor 10. A current of 40 millianrperes peak value decaying to 10 milliamperes appliedvto base 36, of transistor 34 an d a'c'u'rrerit at 100 millianipe'res "decaying to 20 milliamperes -is applied to base 64 of transistor 62, the

output stage. If the output stagcis driven fromcutoff s to saturation by, the application of the b'ase drive, the output current pulse is 400 rnilliampe'reszat a 25% duty cycle. The rise and fall times, are 0. 5 microseconds and the "storage is abou't 0l l 'lnieroseco nd. The rise and-fall times, pnlsewidth-and-storage had an averagedevia'tion of less than '0; 1 "microsecond "random itransistorsj were substituted inthc circuit. 7

Tounderstand the operation of the inductance compensating circuit comprising resistance and capach tance '82; the following prbpos'edexplanation is subrnittetl.

The total dissipation-of a switching transistorconsists of three partsfth'e hase to-emitte'r dissipation, the cnlle'ctor to emitter dissipation during the pedestal portion of the pulse and the eolle'c'tor-to-erriitterdissipation during the transit "time-onifull on,='to full off peridds.

Considering thetlia'grams of FIGSJ and 3' In the lastexpression, T Tis transit time, fispulserepetition frequency -and B is themaximumpower in the load.

Iin FIG. 4, which is a theoretical equivalent circuit of the output stage in FlG. 1, the ideal switch represents transistor 62, C represents capacitance 82, .R represents resistance 80 and R and L representresistance- 86 andinductance 84 respectively. i

Considering the circuit of FIG. 4 :in connection with the expression forP since the pulsefrequencvand collector current will beheld constant; the vfinal expression shnplifies-using K as a 'con'stant.

Capacitance C is inserted "toi'provide partial compens'a 7 "tion for the current rise time determined by L/(Ig-FR). -'I'h eexpression for the "current as; a 'funct'io'n of time yieldsthetwo limiting cases.

When

Using the inverse Laplace transform and solving the re- 7 suiting equation for the critically damped case Expression B may be re-written interms of only the original parameters R, R and L if there is substitutedfor C, its value from expression D.

. A: L Examination of Equation F shows that I'(t) =0 only at 2 equal to infinity thus proving that there is no overshoot. Substitution of the values 0 and infinity for This is the dual analog of the voltage across a shunt 'peakedvideo amplifier interstage' driven by a current source, In this case'the 10% to 90% rise time this particular circuit is one embodiment for providing being continuously variable with neither maximum nor minimum from 1.57 1.75 R T to T as E varies from 0 to 1 The final expression for the critically damped condition 1 R I (nr ir)] (3) The expression for C for this critically dampedco'ndb tionis I Equation 3 can be simplified for a given range of Vai ues including the circuit values given hereinabove to wherein T =rise timeat the 10% to points and K is a constant which was determined experimentally to be 1.77. (In the absence of C, it would be 2.2.) Combining Equations 2 and 5 and defining a new con stant, the formula of Expression 2 reduces to P,, =K L where Equation 6 shows that to drive a load, which has a certain value of L, with a specified current and frequency, there is a corresponding value of transit time dissipation. And with this same load for each value of R there is a corresponding value of C which gives critical damping (4). If, in addition to the load, the rise time is specified, R and its corresponding value of C will be fixed, i.e. Expression 5. The expressions derived herein are based upon an ideal switch and are independent of the characteristics of the transistors. Q

When the ideal switch in FIG. 4 is replaced by a switching transistor as used in the output stage of the circuit of FIG. 1, the rise time of the current through the transistor with no inductance in the load is the limiting minimum rise time that can be obtained. If a chosen value of output current and pulse frequency is specified, the maximum dissaption rating of the transistor dictates themaxirnum inductance of a load that can be-tolerated. Since the voltage drop across the transistor when it is conducting at saturation is a small fraction of a volt, the output current multiplied by the load resistance determines the supply voltage. Equation 5 shows, that with a fixed load, the rise time is inversely proportional to R Therefore the maximum voltage rating of the transistor will dictate the minimum rise time that can be achieved in the cases where the rise time is not transistor limited.

Thus it is seen that with this invention, there is pro; vided a transistor circuit wherein a current "pulse of a chosen amplitude may be supplied through a given inductive load, said pulse having specified rise and, fall times. Also, this invention further provides such a circuit wherein there is substantially reduced to a minimum, any charge storage delay effects.

The circuit of FIG. 1 has been depicted with three amplifier stages in cascade. It is to be understood that a chosen output. Thus, there may be situations Where only one or two stages may he required.

In considering the type of transistor to be used in the output stage of the circuit of FIG. 1, it is important to consider the capabilities of the transistor. As explained above,

' invention;

'ing of the. transistorshould be Qinaccordanceawith the ex pression,l(R +R) wherein I' is peak current, and .R and .lR represent lthe Nalues of resistances 8i) and Y86 respectively. ,Thc' rise. time of the output current pulse is .in

' accordance with theexpression L V p RH whereinListhevalue-of inductance 84 and R and R are the respective values ofxres'istanc'es 80 and 36.

Whil'eihere'havefbecn described what are at present considered'to be the preferred embodiments of the in fvention, it will be obvious "to those skilled in the art that various changes and modifications may be made therein Without departing from the invention and it is,

v threfore, aitne'dto cover a'llsuch changes and modifications as fall within th'e true spirit and scope of the Whatis *claimed'is: ,7

i 1. A transistor circuit for providing a chosen current through an inductive load'comprising an output transistor having said inductive. load, a parallel combination of a resistance and a capacitancein series with said iloadythe value ofisai 'clficapacitance "being "in "accordance with'the wherein TC the warmer said capacitance, L is "the inductance of said loadQR is the value oftheresistiveicom- ,ponent ofisaid load, andR is the valuejot:saidresistance, Lthepowerdissipation of said output transistor being in accordance with the expression I i ssistoriszconducting at saturation and 1 is the average current through the load, the voltage rating of said tranwherein'lT. is transit time, f is pulse irepetitionlirequency, P is .theimaximum power .in.the load when said tran- .sistor heingin accordance with the expressionfI,(R |-,,'R)

wherein ..R and;R:.have their previous significance and lI is peahcurrent,saidrise time being a function or the expression means "for applying biasing potentials to said output itrans'istor whereby said transistor is maintained at collector current cutoff during quiescent statesand wherein said transistor is rendered conductive at saturationfupon the application of an appropriate input signal thereto.

2. Av transistor circuit forproviding a chosen current .ihthrough' an inductive load comprising an output .trant llsistorfhaving saidinductive load, a parallel combination v :oflare'sistanceand a capacitance inseries with said load,

thevalue or said capacitance being in accordance with rthecxpression wherein C is the value -o t capacitance, L is the inductance of said load, R; is the value of the resistance of r;

"whereinL, R and R' have'theirprevious "significance, and

s I I the resistivecomponentrofsaid load R is the Nellie of said resistance, the power dissipation of said output transistor being in accordance the'expression wa VJ wherein T is transit time, 1 is pulse repetition frequency,

P is the maximum power in the load when said transistor is conducting at saturation, and 1, is the average current through the load, the voltage rating of said transistor being in accordance with the expression I(R+R wherein R and R have their previous significancecand l r is peak current, said rise time being a function of the expression V wherein'l, "1r, and R have their previous significance,

means for 'applyinglbiashigpotentials to said output 'trrnrj a v sistor. whereby said transistor is maintained at collector current cutoff during uiescent states, and wherein said transistor is lfilltlfllfidQQlldllCfiQd ;at:.satiiration upon the application of an appropriate input signal thereto, wave shaping means in circuit with the input oisaid transistor and means for applying a rectangular cur-rent wave as an input to said .transistor throughmaid :wave shaping means, said wave shaping means converting "said rec'- tangular wavezto a wave having a relatively steep e tspotential decay, the value of the amplitude of said wave being so chosen that at itslowest amplitude, it is sulficient Ito, causeisaid transistor 1 to conduct at saturation. f

3. Atrans'istor circuit for providing a chosen current through an inductive load comprising an output transistor having said inductive load, a first parallel morn bination of a resistance and a capacitance in series with said load, the value .IOf said wapacitaneebeing in accordance with the expression wherein C is the value of said capacitance, L is the in- 4 ductance of said load, Ris the value of the resistance of the resistive component of said load and R is the value.

of said resistance, the power dissipation of said output transistor being "in accordance "with the expression wherein T is transit time, 'fis pulse repetition frequency, P is the maximum power in the load when said Itransister is conducting at saturation, andl, is theaverag'e current through the load, the voltage'rating of saidttransistor being in accordance with the expression I(R+R wherein R and R have their; previous significance, and I is peak current, said rise time being a function of the expression 7 e PL R +R wherein L, .;R; and "R "have "their ;previous significance, means for applying "biasing potentials to "said output transistor whereby said transistor is maintained at collector current cutofi duringquieseent states and wherein said transistor is rendered conductive at saturation upon the application of an appropriate input signal therctona second parallel combination of a resistance and acapacitance in series with the'input to said output transistor, the values of said resistance andtcapacitance being so chosen as to convert a rectangular wave to a' wave f having a relatively .steep,exponentiahdecay, and means for applying a rectangular wave to .the input of said output transistor through said second,paralleltcornbination,

the amplitude of said wave being so chosen that at most decayed point it is of asuflicient magnitude to maintain said output transistor at conduction at saturation.

4. A transistor circuit for providing a chosen current through an inductive load comprising an output transistor having said inductive load, a first parallel combination of a resistance and a capacitance in series with said load, the value of said capacitance being in accordance with the expression wherein C is the value of said capacitance, L is the inductance of said load, R is the value of the resistance of the resistive component of said load and R is the value of said resistance, the power dissipation of said output transistor being in accordance with the expression wherein T is transit time, F is pulse repetition frequency, P is the maximum power in the load when said transistor is conducting at saturation, and I, is the average 1 current through the load, the voltage rating of said transistor being in accordance with the expression I(R+R wherein R and R have their previous significance and I is peak current, said rise time being a function of the expression L R -l-R wherein L, R and R have their previous significance, a second parallel combination of a resistance and a capacitance in series with the input to said output transistor, the values of said resistance and capacitance being so chosen as to convert a rectangular wave to a wave having a relatively sharp exponential decay, means for applying a rectangular wave to the input ofsaidoutput transistor through said second parallel combination, the amplitude of said wave being so chosen thatat its most decayed point, it is of a sufficient magnitude to maintain said output transistor at conduction at saturation, and means for applying biasing potentials to said output transistor to maintain it at collector current cutoff during quiescent periods and to cause its being rendered conductive at saturation upon the application to the input thereof of said wave, said biasing potentials being so chosen as to also substantially eliminate D.C. restoration efiects caused by said capacitance of said second parallel combination.

5. A transistor circuit for providing a chosen current through an inductive load including a plurality of transistors connected in a series circuit arrangement in the grounded emitter configuration, each successive transistor in said series being of an opposite conductivity type than the immediately preceding transistor, said circuit including an output transistor having said inductive load, a first parallel combination of a resistance and a capacitance in series with said load, the value of said capacitance being in accordance with the expression wherein C is the value of saidcapacitance, L is the inductance of said load, R is the value of the resistance of the resistive component of said load, R is the value of said resistance, the power dissipation of said output transistor being in accordance with the expression wherein T is transit time, 7 is pulse repetition frequency, P is the maximum power in the load when said output transistor is conducting at saturation, and l is the average current through the load, the voltage rating of said transistor being in accordance with the expression I (R+R wherein R and R have their previous significance and I is peak current, said rise time being a function of the expression wherein L, R and R have their previous significance, second respective parallel combinations of a resistance and a capacitance in series with the inputs of the transistors in said circuit, the values of the respective resistances and capacitances of said second parallel combinations being so chosen as to convert a rectangular wave to a wave having a relatively sharp exponential decay, means for applying a rectangular wave to the respective inputs of said transistors through said second parallel combinations, the amplitude of said rectangular wave being so chosen that at its most decayed point, it is suflicient to maintain the transistor to which it is applied at saturation conduction, and means for applying biasing potentials to said transistors to maintain them at collector current cutoff during quiescent periods and to cause their being rendered conductive at saturation upon.

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